The long-term objective of this proposal is to understand the molecular and biochemical basis underlying the regulation of cell growth, death and differentiation by the p53 homologs. The focus of the proposed studies is on deciphering the nature and function of the new human p53-like transcription factor NBP functionally analogous to p53. The principal investigator has purified and identified NBP as the recently cloned p53 homolog p63 from Hela nuclear extracts. Three major members of the p53 family, including p53, p73 and p63, have thus far been identified and cloned by several groups. Although they share biochemical activity and sequence homology and can transcriptionally activate the same group of target genes, genetic and cellular studies demonstrate that they play distinct roles in development and tumorigenesis. This suggests that these members are apparently regulated through different mechanisms and/or by different signals. Indeed, the principal investigator's recent studies demonstrate that although both of the p53 regulators p300 and MDM2 also regulate p73, these proteins apparently utilize different mechanisms. Also, the preliminary studies from the principal investigator's laboratory indicate that p63 and p73 are activated by different types of DNA damage agents. Thus, elucidating the mechanisms underling regulation of p63 and p73 is crucial for a better understanding of the cellular network of the p53 family members. In this application, the principal investigator will dissect the molecular and biochemical mechanisms that govern the stability or activity of the p63 protein in response to differentiation and DNA damage signals. Three specific aims are proposed: 1) To determine the regulatory role of p63 in cell differentiation; 2) To delineate the regulation of p63 function by the p53 regulators in response to differentiation signals. 3) To determine the role of p63 during DNA damage. Molecular dissection of the mechanisms underlying the regulation of p63 will shed light on our understanding of how this protein functions in cells. Moreover, this will have important implications for anti-tumor drug development.